In order to properly image ocean bottom cable (“OBC”) data or deep tow streamer data, the surface-related multiple reflections need to be removed first. The desired information is contained in the direct reflections, and the surface-related multiple reflections, or simply “multiples,” constitute noise that tends to obscure the direct reflections, called primary reflections, or simply “primaries.” Several methods have been proposed to do this.
These include methods based on a difference in spatial behavior of primaries and multiples, like Radon transforms. These methods rely on the assumption that the subsurface has an increasing velocity profile with depth. If this is not the case the methods will fail.
Another type of known method, predictive deconvolution, will remove only the surface-related multiples associated with the water bottom.
Multiple prediction and subtraction methods, such as the surface-related multiple elimination “(SRME”) method of Verschuur and Berkhout (1997), which paper is incorporated herein in all jurisdictions that allow it, and multidimensional up/down division methods, like Amundsen inversion (Amundsen, 1999), have the advantage that they (a) do not make an assumption on the subsurface velocity profile below the water bottom; and (b) remove all surface-related multiples. However, applying them to OBC data is difficult requiring additional data and/or wavefield separation. For example, Verschuur and Neumann (1999) and Ikelle (1999) proposed that SRME could be applied to OBC data but an additional recording of streamer data over the ocean bottom cable would be required. For application to OBC data and deep tow streamer data, Amundsen's method requires that the measured pressure wavefield is separated in an upgoing and downgoing wave field. This separation is not straightforward. Moreover, both of these methods rely on a complete coverage of the water bottom or the deep tow depth with receivers. For shallow water OBC data this is not possible because the source signal will saturate the near-offset receivers and no interpolation algorithm exists to interpolate them with enough accuracy. For the Amundsen inversion the far offsets are also important to prevent artifacts in the end result.
Recently the estimation of primaries by a sparse inversion (“EPSI”) method was introduced by van Groenestijn and Verschuur (this 2009 Geophysics paper is incorporated herein by reference in all jurisdictions that allow it). This is a large scale inversion method that makes use of the same forward model as SRME. Like SRME, this method makes no assumption about the subsurface velocity profile below the water bottom. However, thus far the method has been applied to surface recorded (streamer) data only. The difficulty with applying it to OBC data and deep tow streamer data is that the datum for the recorded wavefields is the ocean bottom respectively deep tow depth whereas the surface-related multiples reflect off of the air-water interface. Thus it is not obvious how to relate the recorded data to the multiples one wishes to estimate and subtract from the data, without additional data (as Verschuur and Neumann (1999) and Ikelle (1999) proposed).
Other published methods for correcting OBC data for multiple reflections are briefly summarized next.
In U.S. Pat. No. 6,678,207 to Duren (“Multiple Suppression for Ocean Bottom Seismic Data”), a method is disclosed that uses a decomposition of the pressure and velocity data into an up- and downgoing wavefield.
In U.S. Pat. No. 6,654,693 to Sen (“Angle dependent surface multiple attenuation for two-component marine bottom sensor data”), a method is disclosed that uses a decomposition of the pressure and velocity data into an up- and downgoing wavefield.
In U.S. Pat. No. 6,101,448 to Ikelle, et al. (“Multiple attenuation of multi-component sea-bottom data”), a method is disclosed that uses two components (pressure and velocity) instead of only one component (pressure) to obtain the primaries.
In U.S. Patent Application Publication No. 20100246324 (“Multiple Attenuation for Ocean-Bottom Seismic Data”), by Dragoset et al., a method is disclosed that needs to decompose the wavefield into an up- and down-going wavefield. This is also a prediction and subtraction method instead of an inversion method.
In PCT International Patent Application Publication No. WO2008076191 (“Identification and Suppression of Multiples in Ocean Bottom Seismic Data”), by Stewart, a method is disclosed that uses two components (pressure and velocity) instead of only one component (pressure) to obtain the primaries.
In PCT International Patent Application Publication No. WO2010/0161235 A1 (“Imaging of multishot seismic data”), by Ikelle, methods are disclosed that either use two components (pressure and velocity) instead of only one component (pressure) to obtain the primaries, or work as a prediction subtraction method instead of an inversion method.